Sanitize HTML to remove meta refresh

Using
  <meta http-equiv="Refresh" content="1; URL=http://example.com/">
in a HTML message causes WebView to load the URL in the default browser.
Overriding WebViewClient.shouldOverrideUrlLoading() allows us to cancel
loading this URL. Sadly, I found no way to find out whether the method was
called because of a meta refresh or because the user clicked on a link.

So now we're using HtmlCleaner to parse the HTML and remove all "meta" elements
containing an "http-equiv" attribute with a value of "refresh".